A very successful prior art system for detecting the leakage of fluid from a container is disclosed in U.S. Pat. No. 4,157,656 by L. I. Walle, entitled Leak Detection System, issued June 12, 1979. The leak detection system disclosed therein includes a monitor which employs an axially displaceable piston having a first surface exposed to fluid pressure within the container and having a second surface opposed to the first surface, with a first portion exposed to a reference gas pressure and a second portion upon which is mounted a striker surface. A gas jet tube is mounted in proximity to the striker surface and has an inlet connected to a pressure sensing source and a sensing gas pressure detector. Changes in the relative magnitude of the fluid pressure within the container with respect to the preference gas pressure will cause the piston to alternately block and unblock the outlet of the gas jet tube, providing a pneumatic signal.
The monitor described in the Walle patent works very well and has met with significant success when applied using reference gas pressure sources which are well regulated and in container leakage measurement operations which are conducted at moderate speed. However, when this prior art monitor is attempted to be employed with poorly regulated reference gas pressure sources, it is possible that erroneous pneumatic signals will be produced by the gas jet tube when transient over-pressured conditions take place in the reference gas pressure source line. Alternately, the fast fill of a container being tested, as is sometimes required by production standards, often results in a marginal pressure achievement. When the pressure from the same regulator is applied to both the vessel under test and the balance side of the piston, since the charging pressure to the vessel is switched off while the reference pressure to the balancing side of the piston remains on, the balancing, reference pressure may achieve its full capacity from the pressure regulator which may sometimes be greater than the pressure in the vessel and thus sufficient to unseat the striker surface from the outlet of the gas jet tube, thereby producing an erroneous pneumatic signal.